1. Field of the Invention
The present invention relates to a constant-frequency active filter controlled by a peak current control mode, and more particularly to a constant-frequency active filter which has a voltage step-up unit and an active power factor corrector to adjust an input current and input voltage to the same phase so as to obtain a high power factor.
2. Description of Related Art
Power factor correcting techniques are generally separated into an active mode and a passive mode. The passive mode means that passive elements such as capacitors and inductors are used to improve a waveform and phase of an input current so as to achieve a purpose of power factor correcting. The active mode means that a power factor corrector is used to control an active switching element to be selectively activated and deactivated for adjusting the waveform of the input current to a sine waveform so as to obtain a high power factor.
An active filter often uses a voltage step-up unit between a bridge rectifier and a filtering capacitor. By controlling the active switching element in the voltage step-up unit, the sine waveform input current is obtained and hence the power factor is raised. The voltage step-up unit controlling techniques generally adopt a voltage-follower mode or an error-multiplier mode. However a switching frequency of the active switching element is not constant and has variation corresponding to different voltage levels of the voltage step-up unit, and hence it is difficult for the active filter to conform with a regulation of electronic-magnetic interference (EMI).
Furthermore, a conventional active power factor corrector is controlled by an analog mode, and a circuit structure of the active power factor corrector is complicated. Once resistance and capacitance values in the active power factor corrector are decided, the active power factor corrector can only operate in a specific condition. If the active power factor corrector is expected to operate in different conditions, the resistance and capacitance values need to be changed to match the different conditions. Thus, the power factor corrector is not suitable for immediately improving harmonic distortions and the power factor of the active filter.
To overcome the shortcomings, the present invention tends to provide a constant-frequency active filter controlled by peak current control mode to mitigate and obviate the aforementioned problems.
The main objective of the invention tends to provide a constant-frequency active filter controlled by a peak current control mode, and the constant-frequency active filter has an active power factor corrector that is composed of a voltage control unit and current control unit for raising a power factor.
Other objects, advantages and novel features of the invention will become more apparent from the following detailed description when taken in conjunction with the accompanying drawings.